It is possible that Carstair's first name is "Bumbly". It is unknown whether "Bumbly" is his first name or just a nickname. In the episode The Gateau from the Chateau, Fairfax talks to his mother on the radio and says: "It's me, Gavin ... I'm with Bumbly Carstairs..."

Fairfax has a girlfriend back home called Daphne who occasionally passes message via the wireless, usually ending in 'Yours Faithfully, Daphne'. It is insinuated that she is having an affair with his commanding officer 'Randy' Hargreaves, but he appears too simple to work it out

In the series pilot episode it was told that he and comrade Fairfax had been shot down. They were later found by the Resistance, who forced René Artois (the owner of Café René) to keep them in hiding after the owner of their previous safe house had been shot. There were many failed attempts by the Resistance to get the pilots back to England and they had to remain in hiding throughout the entire show.

Often they would hide in a secret compartment under the bar or in a wardrobe in the bedroom of Rene's mother-in-law. It was under her bed that the pilots hid a wireless radio and the bedknobs would flash whenever there was an incoming signal, much to the old lady's discomfort. Usually, when their hiding place was opened, they would say with a very British accent, 'Hello!' and then they would answer the wireless or do whatever else they needed to do.

As they couldn't speak French, their oft-heard phrase of "What did he/she say?" became a running gag. This phrase often followed enthusiastic head-nodding at what the French characters were saying, as the pilots pretended to understand. They could communicate only with Michelle of the Resistance (and later with British Intelligence Agent "officer" Crabtree), who fortunately could speak fluent English.

One of their many bizarre French disguises involved them being draped in onions. Once, when cycling along the road dressed as French onion-sellers, the two were aware that everyone was regarding them in a puzzled manner. They eventually realised that they were riding on the British, i.e. the 'wrong', side of the road.